When transferring loads from a vessel to another vessel or to some other construction, which might be movable or unmovable relative to the ground, problems arise due to movement of the water on which the vessel floats. Motion of the water subjects the load transfer device, and consequently the load to be transferred, to similar movements. In case the load is carried by a hoisting cable, the water motion will cause a swinging movement of the load as well. Similar problems arise when a vessel is receiving a load, like a helicopter landing on the vessel, a container or other load. Movement of the water causes the vessel to move, which in turn causes similar movement of the location on the vessel which is to receive the load.
Also when the weather conditions are very calm, the above mentioned problems due to local water movement are present. In this respect it is to be noted that although evidently the water is brought into motion strongly by wind, the effects of wind can lag for weeks in water and have influence on water at large distance away from the location of the wind. Even the water might look like very calm, but still being in motion due to wind weeks ago and/or far away. The effect of this on for example marine building operations is that one has to wait for the water to be almost motionless, in case for example a crane with hoisting cable is to be used safely.
With respect to the motions to which a vessel on water is subjected, it is to be noted that a vessel is in fact subject to 6 degrees of freedom of movement, three translational movements and three rotational movements. Using a mathematical approach based on a carthesian coordinate system having an imaginary set of three orthogonal axes—an x-axis, y-axis and z-axis—these 6 movements can be called x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement. It is to be noted, that from a mathematical point of view there are also other equivalent manners to define the 6-degrees of movement in a space, for example the 3 axes used might not be orthogonal with respect to each other or a so called spherical coordinate system might be used. It is just a matter of mathematical calculation to transfer one definition of 6 degrees of freedom of movement into another definition of 6 degrees of freedom of movement. Using the so called carthesian coordinate system and defining the z-axis as extending vertically, the x-axis as extending in longitudinal direction of a vessel and the y-axis as extending in transverse direction of a vessel,
the x-axis translational movement is in practise called surge
the y-axis translational movement is in practise called sway
the z-axis translational movement is in practise called heave
the x-axis rotational movement is in practise called roll
the y-axis rotational movement is in practise called pitch
the z-axis rotational movement is in practise called yaw
GB 2.163.402 discloses an arrangement for open sea transfer of articles between two vessels, which arrangement uses a gantry—having two hingingly connected arms—mounted with one end of the gantry upon a vessel and carrying on the other free end of the gantry a carrying device in the form of a load platform. The load carrying device is space stabilised, it carries a stabilisation sensing arrangement which senses all three rotational and all three translational movements of the load carrying device in space and provides signals so that the gantry can be controlled by jacks and associated control means for compensation of all three rotational movements and all three rotational movements. This arrangement is complex in construction and unable to compensate for local water movements in case the load is carried by a hoisting cable. Also the control for compensation of 6 degrees of freedom of movement is complex. Further, taking into account that the load platform provided with the sensors is due to being carried by a hinging arm (the gantry) at a large distance from the vessel, the rotational movements of the vessel are first increased in magnitude by the arm length and afterwards compensated, which makes the control more difficult.
U.S. Pat. No. 5,947,740 discloses a simulator enabling an operator to reproduce or represent under test conditions phenomena likely to occur. This simulator comprises a platform carried by six+one hydraulic units. The lower ends of the six hydraulic units are fixed in pairs of two in a triangular pattern to the fixed world and the upper ends are fixed in different pairs of two to a simulation platform, also in a triangular pattern. In rest position all the six hydraulic units extend obliquely with respect to the vertical—none of the hydraulic units being parallel to each other in the rest position. These six hydraulic units are actively controlled to move the platform for simulation purposes. The other one hydraulic unit is a vertical one, which essentially carries the load of the platform and is passive, i.e. not controlled. Advantage of this passive central hydraulic unit is that the other six hydraulic units are just for control of movements of the platform and do not need to support the load of the platform. The forces to be exerted for control of the movement of this platform are thus reduced. Although the document does not appear to say so, this simulator is of the type which is used for flight simulators for training airplane pilots. It is known, that this simulator of U.S. Pat. No. 5,947,740 is also used to compensate a passenger transfer platform on a vessel against movement of the water, so that the passengers can walk easily to another vessel or a construction with fixed position without movement of the gangway. The difference between simulator and movement compensator application being essentially in the control. In the compensator application, the control is based on measurements of movement sensors to compensate the six degrees of freedom of movement of the platform for the measured movement. This compensator and its control system are relatively complex and consequently also expensive.